Petrophysics in brief

Petrophysic studies the physical and chemical properties of rocks, soils and fluids. These properties describe the occurrence and behavior of these elements. It is a subfield of geophysics. The properties that geophysicists study are porosity, density, magnetization, electrical conductivity, solid mechanics, thermal conductivity and radioactivity.

The main concern of petrophysics in the oil and gas industry is the behavior of fluids in rocks. They are also concerned with pressure. They measure these properties (resistivity, natural radiation, sonics) during and after the drilling of an oil well with special instruments. The make these measurements down in the hole. Sonics is the speed of sound. With these measurements they can estimate the type of rock and if oil and gas are present.

To make accurate characterizations of an oil or gas reservoir they take measurements of resistivity, density and measure neutrons. From these measurements porosity, saturation and permeability are calculated. They quantify these properties.

Why do they need to quantify these properties? This is how they study an oil field or gas reservoir. That is how they know how much oil is there and basically where it is. Sometimes a fault or dome stops the oil or gas from spreading.

These understand how geologists study these elements an understanding of the properties is in order. Porosity is the volume by unit of an oil formation. The porosity of a formation can vary quite a bit. Dense carbonates can have a very low percentage of porosity. Limestone and evaporates are dense carbonates.

Shales and clay can have up to 40% porosity. The problem is that the individual holes are usually so tiny that the rock is impermeable for fluid to flow through it.

Consolidated sandstones vary from 10% to 15% porosity. Non-consolidated sandstones porosity can be as high as 30%.

Water saturation is a factor in determining whether there are hydrocarbons in a rock formation or not.  It also helps to estimate the amount of existing hydrocarbons in a known reservoir. This is the calculation of the reserves.

Permeability is basically how much liquid a rock can absorb. This is how the rock’s ability to transmit fluids is measured. For a rock to be permeable it must have interconnected fractures. These are also called pores. Thus a relationship between permeability and porosity exists.

Another tool used in oil exploration is the petrophysical model. It’s used to determine reservoir characterization. That is procedure with which to interpret petrophysical data. This data normally involves equations, algorithms, or another mathematical process.  Many times these models have multiple routines. These are usually wireline logs.

There are different types of models. An example is a deterministic model and its calculations include: the shale volume, total porosity, effective porosity, water saturation and permeability.

Usually the model is calibrated using core and production along with other data sets. Although these models are part of many software packages, unique log-analysis problems are common and a model they need their own special models built in order to make correct calculations.